Newly synthesized eukaryotic mRNA molecules, also known as primary transcripts or pre-mRNA, made in the nucleus, are processed before or during transport to the cytoplasm for translation. Processing of the pre-mRNAs includes addition of a 5′ methylated cap and an approximately 200-250 base poly(A) tail to the 3′ end of the transcript.
The next step in mRNA processing is splicing of the pre-mRNA, which occurs in the maturation of 90-95% of mammalian mRNAs. Introns (or intervening sequences) are regions of a primary transcript (or the DNA encoding it) that are not included in the coding sequence of the mature mRNA. Exons are regions of a primary transcript that remain in the mature mRNA when it reaches the cytoplasm. The exons are spliced together to form the mature mRNA sequence. Splice junctions are also referred to as splice sites with the 5′ side of the junction often called the “5′ splice site,” or “splice donor site” and the 3′ side the “3′ splice site” or “splice acceptor site.” In splicing, the 3′ end of an upstream exon is joined to the 5′ end of the downstream exon. Thus the unspliced RNA (or pre-mRNA) has an exon/intron junction at the 5′ end of an intron and an intron/exon junction at the 3′ end of an intron. After the intron is removed, the exons are contiguous at what is sometimes referred to as the exon/exon junction or boundary in the mature mRNA. Cryptic splice sites are those which are less often used but may be used when the usual splice site is blocked or unavailable. Alternative splicing, defined as the splicing together of different combinations of exons, often results in multiple mRNA transcripts from a single gene.
Up to 50% of human genetic diseases resulting from a point mutation are caused by aberrant splicing. Such point mutations can either disrupt a current splice site or create a new splice site, resulting in mRNA transcripts comprised of a different combination of exons or with deletions in exons. Point mutations also can result in activation of a cryptic splice site or disrupt regulatory cis elements (i.e. splicing enhancers or silencers) (Cartegni et al., Nat. Rev. Genet., 2002, 3, 285-298; Drawczak et al., Hum. Genet., 1992, 90, 41-54).
Antisense oligonucleotides have been used to target mutations that lead to aberrant splicing in several genetic diseases in order to redirect splicing to give a desired splice product (Kole, Acta Biochimica Polonica, 1997, 44, 231-238). Such diseases include β-thalassemia (Dominski and Kole, Proc. Natl. Acad. Sci. USA, 1993, 90, 8673-8677; Sierakowska et al., Nucleosides & Nucleotides, 1997, 16,1173-1182; Sierakowska et al., Proc. Nag Acad. Sci. USA, 1996, 93, 12840-44; Lacerra et al., Proc. Natl. Acad. Sci. USA, 2000, 97, 9591-9596); dystrophin Kobe (Takeshima et al., J. Clin. Invest., 1995, 95, 515-520); Duchenne muscular dystrophy (Dunckley et al. Nucleosides & Nucleotides, 1997, 16, 1665-1668; Dunckley et al. Human Mol. Genetics, 1998, 5, 1083-90); osteogenesis imperfecta (Wang and Marini, J. Clin Invest., 1996, 97, 448-454); and cystic fibrosis (Friedman et al., J. Biol. Chem., 1999, 274, 36193-36199).
Antisense compounds have also been used to alter the ratio of the long and short forms of Bc1-x pre-mRNA (U.S. Pat. Nos. 6,172,216; 6,214,986; Taylor et al., Nat. Biotechnol. 1999, 17, 1097-1100) or to force skipping of specific exons containing premature termination codons (Wilton et al., Neuromuscul. Disord., 1999, 9, 330-338). U.S. Pat. No. 5,627,274 and WO 94/26887 disclose compositions and methods for combating aberrant splicing in a pre-mRNA molecule containing a mutation using antisense oligonucleotides which do not activate RNAse H.
Proximal spinal muscular atrophy (SMA) is a genetic, neurodegenerative disorder characterized by the loss of spinal motor neurons. SMA is an autosomal recessive disease of early onset and is currently the leading cause of death among infants. The severity of SMA varies among patients and has thus been classified into three types. Type I SMA is the most severe form with onset at birth or within 6 months and typically results in death within 2 years. Children with type I SMA are unable to sit or walk. Type II SMA is the intermediate form and patients are able to sit, but cannot stand or walk. Patients with type III SMA, a chronic form of the disease, typically develop SMA after 18 months of age (Lefebvre et al., Hum. Mol. Genet., 1998, 7, 1531-1536).
SMA is caused by the loss of both copies of survival of motor neuron 1 (SMN1), a protein that is part of a multi-protein complex thought to be involved in snRNP biogenesis and recycling. A nearly identical gene, SMN2, exists in a duplicated region on chromosome 5q13. Although SMN1 and SMN2 have the potential to code for the same protein, SMN2 contains a translationally silent mutation at position +6 of exon 7, which results in inefficient inclusion of exon 7 in SMN2 transcripts. Thus, the predominant form of SMN2 is a truncated version, lacking exon 7, which is unstable and inactive (Cartegni and Drainer, Nat. Genet., 2002, 30, 377-384).
Chimeric peptide nucleic acid molecules designed to modulate splicing of SMN2 have been described (WO 02/38738; Cartegni and Drainer, Nat. Struct. Biol., 2003, 10, 120-125).
Antisense technology is an effective means for modulating the expression of one or more specific gene products, including alternative splice products, and is uniquely useful in a number of therapeutic, diagnostic, and research applications. The principle behind antisense technology is that an antisense compound, which hybridizes to a target nucleic acid, modulates gene expression activities such as transcription, splicing or translation through one of a number of antisense mechanisms. The sequence specificity of antisense compounds makes them extremely attractive as tools for target validation and gene functionalization, as well as therapeutics to selectively modulate the expression of genes involved in disease.
Disclosed herein are antisense compounds useful for modulating gene expression and associated pathways via antisense mechanisms, which may include antisense mechanisms based on target occupancy. Provided herein are antisense compounds targeting SMN2 for use in modulation of SMN2 splicing. One having skill in the art, once armed with this disclosure will be able, without undue experimentation, to identify, prepare and exploit antisense compounds for these uses.